This is why he's oh so out of his depth when he has to deal with the fallout of Lizzie's feeling depressed about her looks and such. As I've said before, the man was too busy condemning Lizzie for sucking her thumb and thereby risking her teeth growing in crooked that he completely blanked out on why she did so. Since he won't allow himself to remember the fact that he too was once a child who felt as if a bad grade or a cutting remark on the playground was the end of the world, he assumes sight unseen that since his past was the carefree romp that his parents remember differently that Liz has no problems in her life.
Meanwhile, she's trying to understand why a loving Daddy would want to put an ugly appliance that would make her life worse in her mouth because she can't quite allow herself to realize that the loving Daddy she thinks she has doesn't actually exist. For John to be a loving anything, he'd first to have empathy and that's the one thing he won't allow himself to feel because doing so would possibly make him the bad guy.